1. Field of the Invention
The invention is generally related to a gamma-voltage generator, and more particularly, to a gamma-voltage generator configured to provide gamma voltages with different polarities through a same gamma buffer thereof within different frame periods.
2. Description of Related Art
With the rapid progress in video broadcasting and communication technology, liquid crystal display devices have been used as a display screen in many types of consumer electronic products such as the mobile phones, the notebook computers, the personal computers, and the personal digital assistants (PDAs). Since a liquid crystal display panel itself cannot emit light, it is necessary to dispose a backlight module behind the panel to serve as a light source required by the liquid crystal display panel. Moreover, the light transmittance of the liquid crystal panel is determined by the rotational angles of the liquid crystal molecules within the liquid crystal panel. In particular, the rotational angles of the liquid crystal molecules in the pixels are related to the voltage differences between the pixel electrodes of the pixels and the common electrode. Since the voltage (i.e. common voltage) applied to the common electrode is typically fixed, the pixel light transmittance can be controlled by manipulating the gamma voltages applied on the pixel electrodes.
Driving circuits of conventional liquid crystal displays utilize gamma buffers to stabilize the gamma voltages. Ideally, an ideal gamma buffer has no output error. In other words, in view of the ideal gamma buffer, there is no difference between an input gamma voltage and an output gamma voltage. Referring to FIGS. 1 and 2, FIG. 1 is a diagram illustrating relationships between a DEV voltage of a driving circuit using an idealized gamma buffer and each graylevel. FIG. 2 is a diagram illustrating relationships between a root mean square (RMS) of a driving circuit using an idealized gamma buffer and each graylevel. The DEV voltage is defined as a difference value obtained by subtracting the gamma voltage outputted from the driving circuit by a predetermined idealized voltage. Each of the curves 30(1)-30(n) depicted in FIG. 1 represents a corresponding line of pixels of the liquid crystal display, respectively. Each of the curves 32(1)-32(n) also represents a corresponding line of pixels of the liquid crystal display, respectively. In different frame periods, the liquid crystal display outputs gamma voltages with different polarities. The left side and the right side of FIG. 1 illustrate the situations of the liquid crystal display at negative and positive polarity respectively. It should be noted that the voltages with positive polarity are usually defined as voltages that are greater than the common voltage, and the voltages with negative polarity are usually defined as voltages that are less than the common voltage, and the common voltage may be greater than the ground voltage (i.e. 0 volt) or less than the ground voltage.
However, because the driving circuits of the conventional liquid crystal displays utilize different gamma buffers to output gamma voltages for driving pixels, and because different errors exist between the input voltages and the output voltages of different gamma buffers, the display quality of the liquid crystal display deteriorates. Referring to FIGS. 3 and 4, FIG. 3 is a schematic diagram of a driving circuit 50 of a conventional liquid crystal display during a first frame period, and FIG. 4 is a schematic diagram of the driving circuit 50 during a second frame period. The driving circuit 50 has a first gamma buffer 52(1), a second gamma buffer 52(2), a plurality of digital-to-analog converters (DACs) 54(1)-54(n), and a plurality of operational amplifiers 56(1)-56(n). The driving circuit 50 is configured to output a plurality of gamma voltages to a plurality of lines of pixels 58(1)-58(n) in the liquid crystal display, so as to drive the liquid crystal molecules in the pixels to rotate. The first gamma buffer 52(1) receives a plurality of positive polarity gamma voltages, whereas the second gamma buffer 52(2) receives a plurality of negative polarity gamma voltages. The first gamma buffer 52(1) and the second gamma buffer 52(2) buffer and then output the received gamma voltages to the DACs 54(1)-54(n). Thereafter, according to display requirements, the DACs 54(1)-54(n) respectively select and thereafter output one corresponding gamma voltage of the gamma voltages transmitted from the first gamma buffer 52(1) and the second gamma buffer 52(2).
In the above-described first frame period, the odd-numbered DACs 54(1), . . . , 54(n−3), and 54(n−1) output positive polarity gamma voltages, whereas the even-numbered DACs 54(2), . . . , 54(n−2), and 54(n) output negative polarity gamma voltages. Moreover, in the above-mentioned second frame period, the odd-numbered DACs 54(1), . . . , 54(n−3), and 54(n−1) output negative polarity gamma voltages, whereas the even-numbered DACs 54(2), . . . , 54(n−2), and 54(n) output positive polarity gamma voltages.
However, during the first and second frame periods, because the gamma voltages received by pixels of a same line are respectively buffered by the first gamma buffer 52(1) and the second gamma buffer 52(2), whereby the first gamma buffer 52(1) and the second gamma buffer 52(2) have different errors (input voltages versus output voltages), the display quality of the liquid crystal display deteriorates. Referring to FIG. 5, FIG. 5 is a diagram illustrating relationships between the DEV voltage of the driving circuit 50 and each graylevel. The DEV voltage is defined as a difference value obtained by subtracting the gamma voltage the driving circuit 50 outputs to the pixels by a predetermined idealized voltage. A plurality of curves 60(1,−)-60(n,−) depicted in FIG. 5 represent the corresponding curves when the pixels receive negative polarity gamma voltages. A plurality of curves 60(1,+)-60(n,+) represent the corresponding curves when the pixels receive positive polarity gamma voltages. Compared with the idealized curves depicted in FIG. 1, the curves 60(1,−)-60(n,−) and 60(1,+)-60(n,+) depicted in FIG. 5 significantly deviate from the ideal case. Moreover, referring to FIG. 6, FIG. 6 is a diagram illustrating relationships between the RMS of the driving circuit 50 and each graylevel. Each of a plurality of curves 62(1)-62(n) respectively correspond to a line of the lines of pixels 58(1)-58(n). Compared with the curves depicted in FIG. 2, the curves 62(1)-62(n) depicted in FIG. 6 significantly deviate from the ideal case.